The invention relates to a coating composition for a plastic film. More specifically, the invention relates to a coating composition which improves the physical characteristics of a plastic film. Even more specifically, the invention relates to a coating composition which renders a plastic film receptive to inks, even water based inks.
Polyolefin films are very useful as protective packaging because they resist tearing and water penetration. However, a significant drawback to their use is the difficulties encountered in writing, with ink, on the film. Typically, polyolefin films have low surface energies which make them nonreceptive to certain inks, particularly water based inks. The water based inks tend to exhibit bad ink wetting properties, in which they fail to form a complete coating on the film; the ink forms into discrete beads, streaks and splotches on the film. Additionally, water based inks tend to dry slowly, smear easily and rub off once dry. These are serious problems particularly when plastic films are used for shipping articles which are often exposed to moisture and rough handling. To overcome these difficulties written matter is usually applied to a paper label which is then attached to the wrapping but this is inconvenient and presents recycling problems because paper recycling is usually incompatible with plastic recycling.
Various acrylic coatings are known for purposes of improving the heat seal characteristics of a film. These known acrylic coatings are not known to satisfactorily improve the ink adhesion properties of the film.
U.S. Pat. No. 3,753,769 discloses heat sealable acrylic coating compositions for use on plastic films. The described acrylic coating is formed from an interpolymer of an xcex1-xcex2 monoethyleneically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid and mixtures thereof; and neutral monomer esters comprising (1) methyl acrylate or ethyl acrylate; and (2) methyl methacrylate.
Inorganic particulate hot slip agents, such as colloidal silica and finely divided clay, have been described for use in acrylic terpolymer heat sealable coating formulations in amounts ranging from about 20 to 60 wt. % based on the terpolymer in U.S. Pat. Nos. 4,058,645 and 4,749,616. The hot slip agent improves the machineability of the heat sealable plastic films by enhancing the ability of the outer surfaces of the film to be easily removed from the jaws of a heat sealer and preventing the films from sticking to the metal surfaces of packaging machinery.
The acrylic coating of U.S. Pat. No. 4,058,645 is an acrylic terpolymer of methyl methacrylate, methyl acrylate and methacrylic acid. The acrylic coating includes an aqueous-alkali soluble rosin which improves the heat-seal characteristics of the coating and resistance to seal failure.
U.S. Pat.No. 5,188,867 describes a low coefficient of friction acrylic copolymer coating for plastic film which includes finely divided wax in amounts ranging from 30 to 150 wt. % based on the total weight of the copolymer, a finely divided inorganic solid, such as silica in an amount of 5 to 25 % by weight based on the total weight of the copolymer and talc or syloid in an amount ranging from 0 to 1 % based on the total weight of the copolymer.
The invention relates to an acrylic coating for a plastic film which comprises an amount of a particulate sufficient to improve the ink receptivity of the film.
It is an object of the invention to increase the ink receptivity of a plastic film.
It is a feature of the invention to incorporate a particulate into an acrylic coating for a plastic film which increases the ink receptivity of the film.
It is an advantage of the invention that when the particulates disclosed herein are incorporated, in an appropriate amount, into an acrylic coating that the film has increased ink receptivity.
The coating composition of the present invention contains a significant proportion of a binder which is an acrylic polymer or ionomer resin. The acrylic polymer is usually made from an xcex1, xcex2-monoethyleneically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid and mixtures thereof and neutral monomer esters selected from the group consisting of an alkyl acrylate ester and an alkyl methacrylate ester. The acrylic polymer coating is well known in the art and is described in U.S. Pat. Nos. 3,753,769; 4,058,645; and 4,749,616 which are incorporated herein by reference in their entireties. A commercially available acrylic polymer which is appropriate for use in this invention is sold by Valspar Corp under the product designations Acrylic 79XW318A or 89XW055. Other useful coatings which are contemplated include ionomer resins. Isomer resin is typically a copolymer of ethylene and a vinyl monomer with an acid group such as methacrylic acid or acrylic acid such as the kind sold under the name ADCOTE, specifically, ADCOTE 56220, which is sold by Morton International, specialty chemicals group and Michem 4983 which is available from Michelman Corp. Reference herein to the acrylic polymers include the ionomer resin.
An important feature of the invention is the inclusion of ink receptive particulates in amounts sufficient to increase the ink receptivity of the film. Appropriate ink receptive particulates include clathrates, compounds capable of trapping other substances within their own crystal lattice, such as molecular sieves.
Clathrates, specifically molecular sieves, have demonstrated utility in the coatings of this invention. Examples of typical clathrates are described in Merck Index, 11th Ed., page 365 which is incorporated herein by reference. Clathrates described therein include molecular sieves, cyclotriphosphazenes, Dianin""s compound, hydroquinone, cyclodextrines, o-thymotide and deoxycholic acids. See also Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 6 (Wiley-Interscience, New York, 3rd. Ed., 1979) pp. 178-189, which is incorporated herein by reference in its entirety.
Specific molecular sieves contemplated as ink receptive particulates include natural and synthetic zeolites (alumino-silicates) having a three-dimensional framework structure with channels and interconnecting cavities forming pores. Specifically contemplated zeolites include molecular sieve 13X, 3A, 4A and 5A. These zeolites are more specifically described in Fieser and Fieser, Reagents for Organic Synthesis, Vol. 1, 1967, p.p. 703-705 which is incorporated herein by reference in its entirety.
Other materials which have demonstrated utility as ink receptive particulates include carbohydrate polymers such as cellulose, corn starch and cotton fiber and mixtures thereof. Specific carbohydrate polymers include cellulose polymer, carboxy methyl cellulose and hydroxy ethyl cellulose. The carbohydrate polymer usually requires an amount of either the molecular sieve or silica for enhanced ink receptivity. Typically the ratio of carbohydrate polymer to molecular sieve ranges from about 1:1 to 1:2.
The coating composition, typically, includes finely divided wax. The wax can be a natural wax such as paraffin wax, microcrystalline wax, beeswax, carnauba wax, japan wax, montan wax, etc., or a synthetic wax such as hydrogenated castor oil, chlorinated hydrocarbon wax, or long chain fatty amide. When wax is employed it can be present in amounts ranging from about 2 to about 40% by weight based on the total weight of the acrylic polymer and preferably from about 4 to about 20% by weight based on the total weight of the acrylic polymer.
The coating composition can, optionally, contain a finely divided inorganic solid material such as silica, talc, diatomacious earth, calcium silicate, bentonite and finely divided clays in amounts ranging from about 10 to about 60% by weight based on the total weight of the acrylic polymer, specifically from about 15 to about 50% by weight based on the total weight of the acrylic polymer.
Alkylated melamine crosslinking agents can also be used in the coating composition. These crosslinking agents are generally partially methylated melamine formaldehyde and polymeric and have a degree of polymerization of about 1-3. The following are typical melamine crosslinking agents that are commercially available from American Cyanamid Company: CYMEL 373, a polymeric partially methylated melamine formaldehyde resin having a degree of polymerization of 2.3; CYMEL 385, a polymeric partially methylated melamine formaldehyde resin having a degree of polymerization of 2.1. The following crosslinking agents are available from Monsanto Chemical Company: RESIMINE 714, a polymeric partially methylated melamine formaldehyde resin; RESIMINE 730 and 731 polymeric partially methylated melamine fomaldehyde resins with a degree of polymerization of 1.6; RESIMINE 735 and 740, polymeric partially methylated melamine formaldehyde resins having a degree of polymerization of 1.7 and 1.6, respectively. Another commercially available crosslinking agent is PAREZ 613, a methylated melamine formaldehyde, available from American Cyanamid Company. Other materials that can be employed for crosslinking include glyoxal and borax. The crosslinking agent is usually used in an amount ranging from about 2% to about 50% by weight based on the entire weight of the acrylic polymer, specifically from about 10% to about 30% by weight based on the entire weight of the acrylic polymer. An acid catalyst is usually used along with the crosslinking agent. Appropriate acid catalysts include ammonium sulfate, ammonium chloride, sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid and ammonium nitrate, etc. The acid catalyst is usually employed in an amount ranging from about 0.5% to about 3% based on the entire weight of the acrylic polymer.
The coating composition of this invention can be applied to the surface of the polyolefin film in any appropriate manner. The known techniques for applying coatings to films include gravure coating, roll coating, dipping and spraying and any of these techniques would be appropriate for coating the film of this invention. The coating can be applied to the film after it is formed, usually after it has been oriented by stretching in at least one direction.
Typically, the film is formed by extruding a polyolefin resin, such as polypropylene, through a flat sheet extruder die at a temperature ranging from between about 200 to about 250xc2x0 C., casting the film onto a cooling drum and quenching the film. The sheet is then stretched about 3 to about 7 times in the machine direction (MD) orienter followed by stretching about 5 to about 10 times in the transverse direction (TD) orienter.
Either surface of the film can be treated by surface treatment which improves wettability and adhesion such as by flame or corona treatment.
Typically, the surface of the film, to which the acrylic coating of this invention is applied, is primed so that the coating will strongly adhere to the film. There are many known priming methods. One such method employs polyethyleneimine.
The coating composition of this invention can be applied onto the surface of the film from a non-aqueous solution of the composition using, for example, various organic solvents such as alcohols, ketones, esters and the like. However, since the coating compositions contain particulate materials and since these materials can be difficult to keep well dispersed in organic solvents, it is usually applied in an aqueous solution and often from an alkaline aqueous solution.
The coating composition comprises from about 90 to 30 wt. %, usually 80 to 40 wt. % binder (the acrylic polymer or ionomer resin) and about 10 to 70 wt. %, usually, at least 20 to 60 wt. %, of the above-defined ink receptive particulates.
The coating composition of the present invention is usually prepared to have a total solids content of from about 10% to about 50%, a pH ranging from about 6 to about 10 and a viscosity (Brookfield) of about 10 to about 2500 cP. The coating weight, typically, ranges from about 1 to 5 g/msi (grams per thousand square inch).